Switching arrangement for four-wire circuits



Feb 25, 1936 1 EsPENscHlED Er A1. 290333696 SWITCHING ARRANGEMENT FORFOUR-WIRE CIRCUITS Filed Maron 2, 1935 5 sheets-sheet 1` INVENTORS BY WATTORNEY Feb 25, 1935 L. EsPENscu-HED Er AL Y 2,031,696

SWITCHING ARRANGEMENT FOR FOUR-WIRE CIRCUITS Filed Marh 2, 1955 5Sheets-Sheet 2 Terminal of P/zysicaL bw'- wife Circuit INVENTORSATTORNEY Feb 25, 1936- n...y ESPENSCHIED ET AL SWITCHING ARRANGEMENT FORFOUR-WIRE CIRCUITS Filed March 2, .1955 3 Sheets-Sheet 5 Jwdchz'mgCil-calli Patented Feb. 25, 1936 UNITED STATES PATENT OFFICE sWrroHINGARRANGEMENT Fon FOUR-WIRE CIRCUITS tion of New York Application March 2,1933, Serial No. 659,410

18 Claims. (Cl. 179-15) This invention relates to communication systemsand more particularly to arrangements for switching physical o-requivalent four-wire telephone circuits such as those in use today forlong-haul service.

The present tendency in long distance telephone circuits is to employseparate transmission paths for transmission in opposite direc- 1 tions-The-se separate paths for opposite directional transmission may bephysically distinct wire circuits as in the case of four-wire voicefrequency transmission or they may be paths of different frequenciesderived froma single physical circuit as is done in carriertransmission. The

latter type of transmission has sometimes been called equivalentfour-wire operation.

It becomes necessary for operating purposes at toll centers and otherswitching points to inter- C, connect these various types of longdistance circuits. This is ordinarily accomplished in the case of thevoice frequency circuits by reducing them to two-Wire circuits by theuse of a hybrid coil and balancing network. In the case of the carrierfre- 25 quency circuits the opposite directional transmisso-n paths areconverted to voice frequencies and reduced to a two-wire circuit bymeans of a hybrid coil. Both of these arrangements require that carefulattention be paid to balance which is 3o sometimes extremely difficulttol secure.

The present invention involves the introduction into the four-wirecircuit at its terminal of simple frequency translating, and selectingapparatus which provide means for superimposing the two 5 s'des of thefour-wire circuit at different frequencies upon a simple two-wirecircuit. Switching and monitoring may then be done onV a simple two-wirebasis. An additional advantage is obtained by the elimination of thevproblem of balance, the singing path through the cord circuit beingentirely eliminated.

One object of the invention is thus to interconnect any two of aplurality of four-wire circu'ts by a two-Wire circuit over whichtransmissic-n in one direction is effected at one frequency andtransmission in the opposite direction at a different frequency.

A further object of the invention is to interconnect any channel of acarrier telephone system with any channel of a second carrier telephonesystem by translating the opposite directional frequencies of eachchannel to two different common frequencies.

55 Still a further object of the invention is a common arrangementwhereby, by means of a two-wire circuit, one carrier channel may beconnected to another carrier channel; a carrier channel to a physicalfour-wire circuit or a physical two-wire circuit; or a physicalfour-wire circuit to another physical four-wire or two-Wire circuit.

Yet another object of the invention consists in interconnecting a groupof communicationv channels operated at different frequencies over' oneline circuit with a second group of channels opf-I erated at a secondset of different frequencies over a second line circuit by means of asingle twowire circuit in which is included apparatus for segregatingthe several channels of the incoming group of circuits and translatingthe frequencies of the several channels to the operating frequencies ofthe channels of the outgoing group of circuits with which they are to beconnected.

These and other objects and aspects of the invention will be more fullyunderstood from the following description when read in connection withthe accompanying drawings in which Figure 1 is an arrangement forconnecting two physical four-Wire circuits by employing differentfrequencies in the switching circuit for the different directions oftransmission, while Fig. 11a., when read in conjunction with Fig. 1,shows how the same arrangement may be used for connecting a physicalfour-wire circuit to a physical two-wire circuit. Fig. 2 shows analternate arrangement for connecting together two physical four-wirecircuits in which the principle of frequency translation is employed toprovide a two-wire switching circuit. Fig. 3 shows an arrangement forcon- 35 necting together two equivalent four-wire or carrier circuits inwhich the opposite directional frequencies of the two carrier circuitsare converted to two different common frequencies for transmission overa two-wire switching circuit. Fig. 4 shows an arrangement for connectinga carrier or equivalent four-wire circuit to a physical fourwirecircuit. Fig. 5 is a method for interconnecting physical four-wire,carrier or physical two- 5 wire circuits, which is independent of thetypes of circuits to be interconnected. Fig. 6 shows a method forinterconnecting a group of equivalent four-wire or .carrier circuitsoperating over one line with a second group of carrier circuitso-perating over another line, the second group employing differentfrequencies from the first group.

For connecting two physical four-wire circuits, the arrangement shown inFig. 1 may be employed. This shows two physical four-wire circuits withtransmitting lines TL and TL' and receiving lines RL and RL',respectively, which may be connected to a switching circuit through thejacks and plugs J, P and J', P'. The transmission in one direction overthe switching circuit takes place over a voice frequency path whiletransmission in the opposite direction is stepped up in frequency bymeans of a modulator M, passed over the switching circuit, reduced tovoice frequencies through the demodulator D andV passed into the line.For talking and monitoring a circuit is taken off from the switchingcircuit and connected through the hybrid coil HY to filters and thenecessary modulating and demodulating equipment.

The circuit shown in Fig. 1 is designed to operate as follows: From thereceiving line RL of the physical four-wire circuit shown on the leftside of the figure, transmission passes through the modulator M which issupplied with a carrier frequency from source G through the onewaydevice A whereby the voice frequencies are stepped up to some modulatedcarrier frequency above the voice range. The transmission then passesthrough the filter HP, jack J and plug P to the common switchingcircuit, thence through plug P' and jack J to the high pass filter HP'and the demodulator D, at which point it is reduced to a voice frequencyagain and transmitted to the transmitting line TL' of the four-Wirecircuit shown on the right-hand side of the diagram. Transmission in theopposite direction is accomplished from the line RL' through the lowpass filter LP', jack J and plug P', over the common switching circuitto the plug P and jack J and the low pass filter LP and out to thetransmitting line TL, the high pass filters HP and HP' preventing thetransmission from taking the wrong paths.

For purposes of talking and monitoring, a circuit is taken off thecommon switching circuit and connected to a hybrid coil HY with itsbalancing network N. For talking over the line TL, speech passes fromthe transmitter T through the low pass filter LP1 and hybrid coil HYthrough the filter LP and thence to the line TL. For transmitting in theopposite direction the Voice frequencies from the transmitter T aremodulated by means of modulator M1, passed through the high pass filterHP1 and the hybrid coil HY and thence through filter HP' to thedemodulator D where they are reduced to the voicer frequency range andpassed into the line TL'. The modulator M1 is supplied with carrierfrequency from the source G through the one-Way device A1. Formonitoring, speech coming from line RL is modulated as before bymodulator M with its associated carrier frequency supply, passed throughthe high pass filter HP and through the hybrid coil and high pass filterHP2 to the demodulator D1 where it is reduced to voice frequency againand picked up in the receiver R. For monitoring speech currents from theopposite direction, the voice frequencies from the line RL' are passedthrough the low pass filter LP', the hybrid coil HY, and low pass filterLP2, respectively, into the monitoring receiver R.

It may also be desirable to employ the switching circuit shown in Fig. lfor connection to a two-wire circuit instead of a four-wire circuit.This may be readily accomplished by employing at the terminal of thetwo-wire circuit a hybrid coil arrangement. This is illustrated in Fig.la which may be substituted for that part of Fig. 1 to the right of thedotted line @L -a. The hybrid coil arrangement HY' is substituted forthe physical four-wire circuit TL' and RL', thus permitting thefour-Wire circuit at the left of Fig. 1 to be connected to the two-wirecircuit TW shown in Fig. 1a.

The arrangement of Fig. l requires that modulating, demodulating andfilter equipment be permanently connected into the transmitting andreceiving branches of the physical four-wire circuits. This could beavoided by the insertion of jacks and plugs in each branch of thefourwire circuits at the points X in Fig. 1 and omitting the jacks andplugs J, J' and P, P'. To simplify operation four-Way jacks and plugscould be used.

Another arrangement for connecting together two physical four-wirecircuits is shown in Fig. 2. In this arrangement a two-wire connectingcircuit is employed, the switching circuit being arranged to connect tothe terminals of the fourwire circuits through jacks and plugs J11 andP11 and J12 and P121.

The operation of the circuit of Fig. 2 is as follows: Transmissioncoming in over the branch RL11 of the four-wire circuit shown on theleft of the figure is modulated by modulator M11 which is supplied witha source of carrier frequency from the generator G11 through the onewaydevice A11 which may be a vacuum tube amplifier. The modulated highfrequency passes into the switching circuit through jack J11 and plugP11 and is prevented from going through the lower branch of theswitching circuit by the low pass filter LP12. Accordingly, it passesthrough the high pass filter HP11; and the demodulator D11, in which itis dropped to the voice frequency range. The voice frequency currentsare passed by the filter LP13 through plug P12 and jack J12 and filterLP11 into the line TL12 which forms the outgoing branch of a secondfour-wire circuit. Transmission in the reverse direction is accomplishedfrom circuit RL12 to circuit TL11 in an exactly analogous manner throughmodulator M12, high pass filter HP11, demodulator D12 and low passfilters LP12 and LP11. The modulators M11 and M12 are supplied withcarrier frequency from the source G11 through one-way devices A11 andA12. These devices are for the purpose of preventing coupling betweenthe various branches of the circuits. The one-way devices A13 and A14serve as alternate arrangements for supplying carrier frequency to themodulators M11 and M12 in which case the one-way devices A11 and A12 andtheir associated wiring may be omitted.

In case it is desired to talk and monitor over the cord circuit this maybe accomplished by means of a monitoring circuit similar to that shownin Fig. 1 which may be connected as indicated in Fig. 2.

In the arrangement shown in Fig. 2, it may be desirable to employ twodifferent carrier frequencies instead of a single carrier frequency andthe voice frequency circuit. This may be readily accomplished by theintroduction of additional demodulators and the proper filters.

The arrangements so far described have been for the purpose ofconnecting together two physical four-wire circuits or a physicalfour-wire circuit to a physical two-wire circuit. Where it is requiredto connect together two carrier or equivalent four-wire circuits, thearrangement of Fig. 3 may be employed.

In this arrangement the carrier frequencies assigned to the transmittingchannels of one of -flO the `carrier systems are changed in frequency tosome common frequency differing from the Yfrequencies used by vany ofthe carrier systems to be connected together. `The 'receivingA carrierfrequencies of the receiving channels of a second carrier system towhich the first is to be connected are derived by suitable translationof the common frequency to which the first carrier frequency channelswere changed. Similarly the transmitting channels of the second systemtransmitting in the reverse direction are shifted to another commonfrequency, and the frequencies of the receiving channels of the firstcarrier system are derived therefrom. Two channels operating ondifferent frequencies may thus be connected together. By raising thefrequencies. of all of the Vchannels of the carrier systems to beinterconnected to two common frequencies, one for transmission in onedirection and one for transmission in the reverse direction, any channelof any carrier system may be connected to any channel of any othercarrier system. By means of additional filters, modulators anddemodulators connected to the switching circuit as shown in Fig. 1,talking and monitoring over vthe switching circuit may also beaccomplished.

The operation of the arrangement of Fig. 3 may be explained as follows:It is desired to connect channel l of the carrier system operating overthe line A21 with a channel of another carrier system operating over theline B21. Communication is accomplished over channel I of the firstcarrier system by the use of two different frequencies F1 and F2, fortransmission in opposite directions, While transmission over the carrierchannel of the second system is effected by the use of two carrierfrequencies F1 and Fa which may be the same as frequencies F1 and F2 orthey may be different frequencies. These carrier frequencies may be anyfrequencies suitable for carrier frequency transmission. The incomingvoice modulated carrier frequency currents from line A21 are selected bythe band filter BFi and raised in frequency by the modulator M21, whichis supplied with a source of carrier lfrequency G21 to some frequencyF31=F1+G21 well above the frequencies normally used for carriertransmission. The modulated high frequency current F31 including oneside band is selected by filter BF31 and passed through jack J21 andplug P21 and passed over the switching circuit C21 and thence throughplug P22 and jack J22 and band lter BF31 into the modulator M21. M21' issupplied with a source of carrier frequency G21 such that when combinedwith the incoming modulated frequency F311, the Yfrequency F7 isproduced. This frequency and one of its accompanying side bands isselectedv by the band filter BF2 and passed on to carrier line B21.

Transmission is eifectedfr'oin carrier -line'B21 to carrier line A21 ina similar manner, the common modulated carrier frequency F32 and itsaccompanying side band being diiferent from that used for transmissionin the opposite direction.

By employing suitable frequencies in connection with modulators M23,M24, M25, M23, M23', M24', M25', M23', transmission in one directionthrough the switching circuit maybe effected by the common frequency F31and transmission in the opposite direction by the common frequency F32.Thus any channel of any carrier system so arranged may be connected toany channel of any other carrier system by means of the circuitillustrated. For example, by inserting plug P21 in jack J23 and plug P22in jack J2e, transmission maybe effected from channel 2 of the carriersystem indicated on the left of the figure to channel 3 of the carriersystem on the right of the ligure. n

Although in the arrangement shown in Fig. 3 the frequencies used fortransmission in opposite directions through the switching circuit aredifferent carrier frequencies F31 and F32, it would be possible toemploy the Voice frequency range for one direction of transmission, saythe direction from east to west, by assigning such frequencies to thegenerators G22', G24', and G26 as to step the received carrierfrequencies to Zero. The frequencies assigned to generators G22, G24 andG23 will then be F2, F4 and F3. The lters BF32 and BF32' will then bedesigned to pass the voice band.

Still another arrangement employing the principles outlined in thisinvention may be used for connecting a carrier or superimposed fourwir'ecircuit to a physical four-wire circuit. An arrangement for doing thisis shown in Fig. 4. Modulated carrier frequency coming in from thecarrier line is selected by the proper lter, demodulated, passed throughjack J31 and plug P31 and over the switching circuit C31 through plugP32 and jack J32 and low pass filter LP31 to the outgoing line of thephysical four-wire circuit. Transmission in the opposite direction iseffected by modulating the Voice frequencies coming from the physicalfour-wire circuit, transmitting them as high frequencies over theswitching circuit C31 and again modulating the high frequency currentsto a frequency which is the same as that of the outgoing carrierfrequency. l In more detail the operation of this circuit is as follows:The incoming modulated carrier fre'- quency current corresponding tochannel I of a carrier system operating over line A31 and ernployingcarrier frequencies F31 and F32 is selected and passed by the bandfilter BF31, demodulated to a voice frequency by the demodulator D31,transmitted through jack J31 and plug P31 and the switching circuit C31through plug P32 and jack J32 and .low pass filter LP31 to the outgoingline TL31 of the physical four-wire circuit. ASpeech coming in from theincoming line' RLai of the physical four-wire circuit is modulated bymeans of modulator M34 with a high frequency supplied by the generatorG37 and passed through jack J32 and plug P32, the switching circuitC'31, plug P31 and jack J 31 into modulator M32 where it is againmodulated by a carrier frequency G42 of such a value that theinteraction of frequencies G42 and F37 provides a frequency F32. Themodulated carrier frequency F32 and its appropriate side band areselected by band filter 3F32 and passed through it to the carriersystem. The frequency of the generator G32 may be chosen to be of suchValue that it lies outside the range of frequencies normally used forcarrier transmission.

Here again, although the frequency bands used for illustration in Fig. 4for transmission in opposite directions through the switching circuitconsist of a voice frequency band and a carrier frequency band, it wouldbe possible `to employ two different carrier frequency bands differingin turn from any band employed by the carrier systems. In this case, theband filter BF33 and the demodulator D34 which are shown in Fig. 4 wouldbe included in the outgoing branch of the physical four-wire circuit.

Still another arrangement by which it is possible to connect one carrierchannel to another carrier channel; a carrier channel to a physicalfour-wire circuit or a physical two-Wire circuit; or a physicalfour-wire circuit to another physical four-wire or two-wire circuit isillustrated in Fig. 5. The arrangement shown in this figure comprises aVery exible circuit arrangementl by which practically any type ofcircuit may be interconnected to any other type of circuit by a two-wireswitching circuit. The switching circuit employed is, in general,similar to that illustrated in Fig. 2. A talking and monitoringarrangement may be associated with the switching circuit. Such anarrangement, which might be similar to that shown in Fig. 1, could beconnected to the circuit as indicated in the figure.

The operation of Fig. 5 is as follows: Assuming that the carrier channelof line A41 which employs the carrier frequencies F1 and F2 is to beconnected to the carrier channel of line B41 which employs the carrierfrequencies F1 and Fa, the plug P of the switching circuit will beinserted in the jack J41, and the plug P' in the jack J41. The carrierfrequency F1 from the carrier line A41 will then be combined in themodulator M21 with the frequency G21 to produce a frequency F31 whichpasses through the filter HP31 of the switching circuit to thedemodulator D31. The demodulator D31 steps the frequency F31 down to theVoice frequency which passes through the low pass filter LP' and thenceover the plug P' and jack J41 and through the low pass filter LP1' tothe modulator M21'. The voice current is modulated on the frequency F2supplied to the modulator M21', and the carrier frequency F1 and theside band or bands pass through the filter BF7 to the line B41. Intransmitting in the opposite direction the carrier frequency Fa from theline B41 is modulated with the frequency G22 in the modulator M22 toproduce the frequency F32 which passes through the band filter BF32' andthrough the high pass filter HP32 in the lower branch of the switchingcircuit to the demodulator D32. The demodulated voice frequency from theoutput of the demodulator D32 passes through the low pass filter LP andover the plug P in jack J41, and thence through the low pass filter LP2to the modulator M22. In this modulator the voice frequency modulatesthe carrier frequency F2, and the carrier frequency F2 and the side bandpass through the band filter BF2 to the line A41. It will be understood,of course, that other channels of the carrier line A41 will have suchfrequencies assigned to terminal modulators of the line as to translatethe other eastbound carriers to the frequency F31, and likewise, theline B41 will have such frequencies assigned to its modulators as totranslate the other westbound carriers to the frequency F32.Consequently, any carrier channel of the line A41 could be connected toany carrier channel of the line B41 through the switching circuit.

Assuming the carrier channel employing frequencies F1 and F2 of the lineA41 is to be connected to the physical four-wire circuit B51, the plug Pof the switching circuit will be connected to the jack J51 of thephysical four-wire circuit. The eastbound transmission F1 will appear asa voice frequency at the plug P', and passing over the jack J 51 will betransmitted through the filter LP1' to the east-going line of thephysical fourwire circuit B51. The voice frequency from the Westboundline of the four-wire circuit will modulate the frequency F32 suppliedto the modulator M32', and the frequency F32 and its side band will passthrough the filter BF32', thence passing over the jack J51 and the plugP' into the lower branch of the switching circuit from which point theoperation will be as previously described.

In the case of the connection from the line A41 to the two-Wire lineB52, since the two-wire line terminates in eastbound and westboundbranches by means of the hybrid coil HY41, and these two branchescontain the same apparatus as is used in the four-Wire circuit B51, theoperation When the line A41 is connected to the line B52 will beobviously similar to that described in connection with the line B51.

If the physical four-wire circuit A50 is to be connected to the carrierchannel comprising fre- .quencies F7 and F3 of line B41, the eastboundvoice frequency in the upper line of the fourwire circuit will bemodulated in the modulator M31 upon the carrier frequency F31 and themodulated carrier will pass through the band filter BF31 and over thejack J50 into the upper branch of the switching circuit. In theswitching circuit the demodulator D31 again steps the frequency down tothe Voice range, and the demodulated voice frequency passes over thejack J41' and through the filter LP1' of the upper carrier channel ofthe line B41 to the modulator M21'. In the modulator M21' the voicefrequency is modulated on the carrier F7 supplied by generator G7 andtransmitted to the line B41. The carrier F3 from the line B41transmitted in the opposite direction is by means of modulator M22',translated to the frequency F32 which passes into the lower branch ofthe switching circuit and is demodulated by means of the demodulator D32to the voice frequency which passes over the jack J50 and through thelow pass filter LP2 into the lower line of the physical four-wirecircuit A50.

If the four-wire circuit A50 is to be connected to the four-wire circuitB51, the voice frequency after being stepped up to the frequency F31 inthe upper line of the four-wire circuit will pass through the upperbranch of the switching circuit wherein it is stepped down again tovoice frequency. The voice frequency then passes over the jack J51 intothe eastbound line of the physical four-wire circuit B51. The voicefrequency from the westbound line of thev four-wire circuit B51modulates the frequency F32 in the modulator M32 and the modulatedfrequency then passes through the band filter BF32 into the lower branchof the switching circuit where it is stepped down to voice frequency bymeans of the demodulator D32, the voice frequency then passing to thelower line of the physical four- Wire circuit A50.

If the four-wire circuit A50 is to be connected to the two-wire line B52the operation will be similar to that just described.

Still another arrangement which embodies the general principles of theinvention is illustrated in Fig. 6. By means of this arrangement twocarrier telephone systems having different frequency assignments may beconnected together by a switching circuit which includes selecting bandfilters and modulators for changing the incoming frequencies of onecarrier system to the outgoing frequencies of the other carrier systemand the reverse. A number of different filters and modulators areconnected in parallel in the switching circuit, thereby making possiblethe interconnection of carrier lines employing a wide Vvariety offrequencies by the same switching circuit.

Referring to Fig. 6, the operation of the arlll rangement is as follows:The incoming modulated carrier frequency F51 representing one channel ofany one of the carrier lines on the left of the gure, such as L52, istransmitted through jack J52 and plug P51 to the switching circuit.I-Iere it is selected by the band pass iilter BF51 and modulated bymodulator M51. Modulator M51 is supplied by a source of carrierfrequency G51, the frequency of which is equal to the difference betweenthe frequency F51 and the frequency F55, which is assumed to be theoutgoing carrier frequency of the channel to which the incoming channelof frequency F51 is to beV connected. The proper modulation products andthe frequency F57, if necessary, issuingfrom the modulator M51 areselected by the band lterY BF52 and transmitted to the carrier line L55through plug P52 and jack J55. Signals coming in the opposite direction,such as the modulated .frequency F58 from the carrier line L55 aretransmitted through the switching circuit in an exactly analogousmanner, being modulated in the switching circuit by modulator M52 with afrequency equal to the difference between F58 and F52, the frequency F52being the opposite direction channel frequency of the carrier systemcorresponding tothe incoming frequency F51. A pluralityV of band filtersand modulators are connected in the switching circuit as shown, in orderthat all channels of any of the carrier lines shown on the left may beconnected to all channels of any of the carrier lines shown on theright. It should be noted that the frequencies assigned to the carrierlines may be any different frequencies, it merely being necessary toprovide in the switching circuit suitable band filters and modulatorssupplied with frequencies equal to the difference between thefrequencies of the channels which it is desired to connect together.

It will be obvious that the general principlesl herein disclosed may beemployed in many other organizations widely different from thoseillustrated without departing from the spirit of the invention asillustrated above and defined in the appended claims. y

What is claimed is:

1. In a telephone switchingoiiice havingterminated therein apluralityofV physical four-wire circuits, a plurality of carrier lines havingsuperimposed on each a plurality of equivalent four-wire circuits, aplurality of two-wire physical circuits and a switching circuit forinter-connections, the method for interconnecting any circuit to anyother circuit'comprising the translating of the opposite directionalfrequencies" of any pair of circuits to be interconnected to twodifferent common frequencies for transmission over the' switchingcircuit.

2; In a telephone toll switching oflice having terminated therein aplurality of physicalrfourwire. circuits, a plurality of carrier lineshaving superimposed on each a plurality of equivalent four-wirecircuits, and a plurality of two-wire physical circuits, a switchingcircuit tointerconnect any circuit to any other circuit, and means fortranslating the opposite directional frequencies of any pair of circuitsto beinterconnected to two different common frequencies for transmissionover said switching circuit, said switching circuit including means fortalking and monitoring on any circuits to which said switching circuitmay be connected.

3. In a telephone system, means for switching a plurality of two-Waycommunication channels including a switchboard providedlwithterminals ofa plurality of line circuits carrying a plurality of communicationchannels, a pair of terminals for each of said channels available on theswitchboard for making connection thereto, a switching circuit forinterconnecting any two of said channels upon call, said switchingcircuit including two pairs of movable contacts, and means related tosaid switching circuit for translating the incoming frequency band ofeach channel to the outgoing frequency band of the correspondingchannel.

4. An electric communication switchboard comprising a plurality .oftwo-wire line circuits conveying communication channels on particularfrequency bands and eachof said channels appearing on a pair of terminalcontacts, and means for connecting together upon call any two of Vsaidlpairs of contacts, and means for translating the frequency bands of oneof the corresponding communication channels to the frequency bands ofthe other.

5. In a telephone switching oilice, the method for interconnectingfour-wire circuits which consists in translating the frequency band ofthe incoming transmission from one direction at the switching oce tosome common frequency band, translating the frequency band of theincoming transmission in the Iopposite direction at the switching officeto some other common frequency band, transmitting the two differentopposite directional commonV frequency bands over at least a portion .ofa switching circuit and retranslating the two common frequency bands tothe frequency bands employed for the outgoing transmission. l

-6. In a" telephone switching oflice, the method for interconnectingphysical four-wire circuits which consists in translating the frequencyband of the incomingA transmission from one direction at the switchingoffice to some common frequency band for transmission lover a switchingcircuit, translating the frequency band of the incoming transmission inthe opposite direction at the switching office to some other commonfrequency band, transmitting the two different opposite directionalfrequency bands over a switching circuit and retranslating the twofrequency bands to the frequency bands employed for the outgoingtransmission.

7 The method of interconnecting by a switching circuit at a' switchingpoint a plurality of physical four-wire circuits in pairs for two-waytelephone communication, which consists in raising the incoming voicefrequencies from each incoming line at the switching point to somefrequency band above the voice frequency range and translating in theswitching circuit the modulated high frequency currents received fromeach incoming line into voice frequency currents for transmission overeach outgoing line.

8. Inv a telephone switching oiiice in which a plurality of carrierfrequency channels are to be interconnected inpairs for two-waycommunication, the method which consists in raising the incomingmodulated carrier frequencies from each incoming channel in onedirection to some common frequency band different from those used by thecarrier channels, interconnecting one of said channels to an outgoingcircuit in the same direction by Va switching circuit, translating thecommon frequency band so that' in each outgoing carrier channel in thesame direction will appear the frequency band used for transmission overits carrier line, transmission in the reverse direction occurring in asimilar manner but employing a second common carrier frequency bandwhich is different from the first mentioned common frequency band.

9. In a telephone switching office, the method for interconnectingequivalent four-Wire circuits which consists in translating thefrequency band of the incoming transmission from o-ne direction to somecommon frequency band, translating the frequency band of the incomingtransmission in the opposite direction to some other common frequencyband, transmitting the two different opposite directional frequencybands over at least a portion of a switching circuit and retranslatingthe two frequency bands to the frequency bands employed for the outgoingtransmission.

10. In a telephone switching oflice in which are terminated a pluralitylof carrier frequency lines, each having a plurality of two-waycommunication channels operating over it, and a plurality of physicalfour-wire circuits the method for interconnecting any one of saidcarrier communication channels to any one of said physical four-wirecircuits which consists in translating the incoming modulated carrierfrequencies to the voice frequency range, passing the voice frequenciesover a portion of a switching circuit to the outgoing branch of thephysical four-wire circuit; translating the Voice frequencies from theincoming branch of the physical four-wire circuit to some frequency banddifferent from the frequency bands employed by the carrier circuits,passing said modulated' high frequency band over a portion of theswitching circuit and translating said modulated high frequency band tothe outgoing frequency band of the carrier channel.

11. In a telephone switching oiiice, the method for connecting any oneof a plurality of carrier frequency lines, each of which has operatingover it a plurality of communication channels, to any other lof saidcarrier frequency lines which consists in separating the incomingcarrier channels in a switching circuit, and translating the frequencyband of each incoming carrier channel to the frequency band of thecorresponding outgoing carrier channel.

l2. In a telephone switching ofce containing a plurality of physicalfour-wire circuits, means for interconnecting any twoof said circuitscomprising translating means at the switching office for raising theincoming voice frequencies in each incoming branch of said circuits tosome frequency band above the voice frequency range and superimposingthis incoming high frequency branch on the outgoing voice frequencybranch, a switching circuit having two branches each branch includingmeans for selecting and translating each incoming high frequency band tothe voice frequency range and means for connecting each end of theswitching circuit to the physical four-wire circuits.

13. In a telephone switching oice containing a plurality of physicalfour-wire circuits means for interconnecting any two of said circuitscomprising translating means in each incoming branch of said circuitsfor raising the incoming voice frequencies to some frequency band abovethe voice frequency range and superimposing this incoming high frequencybranch on the outgoing voice frequency branch, a switching circuithaving two branches each branch including means for selecting andtranslating each incoming high frequency band to the voice frequencyrange and means for connecting each end of the switching circuit to thephysical four-wire circuits, said switching circuit including means fortalking and monitoring on the interconnected circuits.

14. In a telephone switching oiiice, a plurality of physical four-wirecircuits, each of said fourwire circuits including translating means inits incoming branch for raising the incoming Voice frequency to somefrequency band above the voice frequency range, the incoming highfrequency and outgoing low frequency branches of each of said four-wirecircuits being connected together, aV two-wire switching circuit forconnecting any one of said physical four-wire circuits to any other ofsaid physical four-wire circuits, said switching circuit including meansfor passing the transmission in opposite directions over two separatebranches, each branch including means for selecting the incomingmodulated high frequency band and translating it to Voice frequencies,and means for connecting said switching circuit to the physicalfour-wire circuits.

15. In a telephone switching office in which are terminated a pluralityof carrier telephone lines each line having superimposed thereon anumber of communication channels employing different frequencies for thedifferent directions of transmission, means for terminating each channelin two branches, one branch for receiving and one branch fortransmitting, the two branches being connected together on the officeside, means included in the receiving branch for selecting andtranslating the received frequency band to some other frequency band b1,differing from any frequency band employed on the carrier lines, meansincluded in the transmitting branch for selecting and changing a commonfrequency band b2, differing from any frequency band used on the carrierlines and differing from b1, to that frequency band normally used fortransmission over that particular channel of the carrier line, and meanscomprising a switching circuit for interconnecting any channel of onecarrier line to another channel.

16. In a telephone switching oice in which are terminated a plurality ofcarrier telephone lines, each line having superimposed thereon a numberof communication channels employing different frequencies for thedifferent directions of transmission, means for terminating each channelin two branches, one branch for receiving and one branch fortransmitting, the two branches being connected together on the ofliceside, means included in the receiving branch for selecting andtranslating the received frequency band to some common frequency band,differing from any frequency band employed on the carrier lines, meansincluded in the transmitting branch for selecting and changing anothercommon frequency band, differing from any frequency band used on thecarrier lines and differing from said first-mentioned common band, tothat frequency band normally used for transmission over that particularchannel of the carrier line, a plurality of physical four-wire circuits,means for interconnecting any terminal of any carrier channel to anyterminal of any physical four-wire channel comprising a twowireswitching circuit, the incoming branch of each four-wire circuitincluding a modulator for raising the incoming Voice frequencies to acommon frequency band to said switching circuit, and a demodulator sorelated to each outgoing branch as to translate a common modulatedfrequency band to voice frequencies.

1'7. In a telephone toll switching oiice, a plurality of carrier lines,each line having superimposed thereon a plurality of channels employingdifferent frequencies for the .different directions of transmission,means for terminating each channel in two branches for the oppositedirections of transmission, said branches being connected together onthe ofi-ice side, the incoming branch containing translating means forthe purpose of shifting the incoming modulated carrier frequencies tofrequencies in a common range, and each outgoing branch including meansfor translating another common band to the frequency band of theoutgoing carrier channel, a plurality of four-wire circuits, theincoming branch of each of said four-wire circuits containing atranslating device for the purpose of raising the incoming voicefrequency band to a common carrier frequency band differing fromvanyfrequency band employed on the carrier lines, the outgoing branch ofeach of said four-wire circuits containing voice frequency selectingmeans, said' incoming high frequency and outgoing low frequency branchesof said four-wire circuit being connected together, and switching meansfor interconnecting any carrier channel to any physical four-wirecircuit.

18. In a telephone switching office having a plurality of carrier linesassociated therewith, each line having superimposed thereon a pluralityof communication channels operating on different frequencies, said linescomprising two groups, each line of each group employing the same set ofcarrier frequencies but the two groups employing different sets offrequencies, means for interconnecting all of the channels on any lineof one group to all of the channels on any line of the second groupcomprising a two-wire switching circuit, said circuit including inparallel a plurality of selecting filters and modulators for translatingthe frequency of each channel of one group of carrier lines to thecorresponding channel of the second group of carrier lines.

LLOYD ESPENSCHIED. NEWTON MONK.

